This patent specifcaton relates to the field of optical fibers. More particularly, it relates to a microstructured optical fiber that allows for improved transmission efficiency and physical durability in fiber optic communication applications.
Advances in fiber optics technologies have made optical fiber communications the method of choice in the transmission of high bit-rate digital data over long distances. A conventional optical fiber is essentially an optical waveguide having an inner core and an outer cladding, the cladding having a lower index of refraction than the core. Because of the difference in refractive indices, the optical fiber is capable of confining light that is axially introduced into the core and transmitting that light over a substantial distance. Because they are able to guide light due to total internal reflection principles, conventional optical fibers are sometimes referred to as index-guiding fibers. Conventional optical fibers are made of fused silica, with the core region and the cladding region having different levels of dopants (introduced impurities) to result in the different indices of refraction. The cladding is usually doped to have a refractive index that ranges from 0.1% (single mode fibers) to 2% (multi-mode fibers) less than the refractive index of the core, which itself usually has a nominal refractive index of 1.47.
Conventional optical fibers have a solid cross-section. As light travels through the solid fused silica material, it is subject to several adverse effects that reduce the efficiency of information transfer and the practical distance over which information may be carried by the light. These effects include attenuation or loss (reduction in signal magnitude), dispersion (chromatic, waveguide and modal), miscellaneous nonlinearities (such as stimulated Raman scattering, stimulated Brillouin scattering, and optically induced birefringence), and other adverse effects.
Although the light is being transmitted through many meters or kilometers of solid material, relatively low losses can be experienced at certain wavelengths of light. Conventional fibers today, for example, have attenuations as low as 0.25 dB/km at 1550 nmxe2x80x94about 1% of the light entering the fiber still remains after 80 km. Using today""s amplifier and detector technologies, this allows signals to go through more than 100 km of fiber without amplification, an important advantage in long distance communications. Nevertheless, it would be desirable to even further minimize the above mentioned adverse effects in optical fibers, for increasing the efficiency and reducing the cost of fiber optic communications.
Another problem that arises in conventional optical fibers relates to the physical durability and robustness of the optical fiber itself. A variety of outside influences can change the physical characteristics of optical fibers and affect how they guide light. As a first example, the bending of an optical fiber into tight loops or other tightly curved shapes may cause the unwanted propagation of microcracks in the fused silica structure. Upon such bending of the fiber, the microcracks can become larger and extend across a substantial portion of the cross-section of the optical fiber, rendering it inoperable. As a second example, external forces may squeeze or pinch the outside surface of an optical fiber, such as when a fiber is tightly pulled around a sharp corner. Upon such squeezing or pinching, the structure of the fused silica material inside the fiber may contort slightly. This can cause unwanted polarization effects in the light being transmitted through the fiber, also reducing the fiber""s capacity or rendering it inoperable. Accordingly, it would be desirable to provide a fiber optic structure that is more robust to external bending, pinching, or squeezing of the fiber optic.
As described by Broeng et. al. in WO9964903, recent developments in optical fiber technology have been introduced by way of microstructured photonic bandgap (PBG) fibers. In contrast to conventional optical fibers in which a high-index core is surrounded by a low-index cladding, PBG fibers comprise a low-index (or even hollow) core surrounded by a higher-index cladding that contains carefully placed air voids. The air voids run longitudinally, parallel to the central axis of the fiber. When the air voids are placed in the cladding such that a cross-section of the fiber has a specific, predetermined, periodic pattern of air voids, a photonic bandgap (PBG) effect may be achieved. When the PBG effect is achieved, the cladding structure is capable of completely reflecting certain wavelengths of light at certain incident angles, and is thereby capable of confining the light to a region surrounded by the cladding structure for propagation down the length of the fiber. The PBG effect is achieved even though the refractive index within the region of confinement may be lower than that of the surrounding cladding structure.
PBG fibers, however, contain a crucial shortcoming in that proper operation is based on an interference effect. This is in contrast to conventional index-guiding fibers that guide due to total internal reflection. Because they depend on an interference effect, PBG fibers are extremely sensitive to even slight variations in the locations of the air voids in the cladding. Substantial deterioration in performance may take place if even one of the air voids is slightly misplaced. Even if properly manufactured to exacting tolerances, slight variations in the relative air void positions might be incurred due to external twisting, pinching, or squeezing that slightly deforms the fiber optic structure. This, in turn, may lead to drastic performance decreases. Accordingly, PBG fibers are not used today in practical fiber optic communication systems, although they continue to be the subject of laboratory research.
Another type of optical fiber having longitudinal air voids is presented in U.S. Pat. No. 5,802,236 to DiGiovanni et al., hereby incorporated by reference herein. The ""236 patent discloses a microstructured optical fiber comprising a solid core region surrounded by a cladding region having a plurality of air voids. In contrast to PBG fiber in which careful periodic spacing of the air voids is required, the air voids of the ""236 patent are not required to be periodic. The optical fiber of the ""236 patent relies on index-guiding, and not on the PBG effect, to propagate the light down the fiber, the index-guiding effect being achieved when the effective refractive index of the cladding is less than that of the core. According to the ""236 patent, because a portion of the cross-sectional area of the cladding is occupied by air voids, the effective index of refraction of the cladding region will be less than that of the core, and index-guiding will be achieved. Roughly speaking, the effective index of refraction of the cladding will be an average of the refractive index of air and the refractive index of the fused silica material, weighted according to the percentage of cross-sectional area occupied by each.
FIG. 1 illustrates a cross-sectional view of an optical fiber 100 similar to that disclosed in the ""236 patent and having dimensions as described in the ""236 patent. Optical fiber 100 comprises a core region 102 surrounded by a cladding region 104. The core region 102 is solid glass material. The cladding region 104 is solid glass material surrounding a plurality of air-void cladding features, in particular, first cladding features 106 and second cladding features 108. The first cladding features 106 are positioned such that the inscribed diameter of core region 102 is 1.017 xcexcm. The first cladding features 106 each have a diameter of 0.833 xcexcm, while the second cladding features 108 each have a diameter of 0.688 xcexcm, the cladding features all having a center-to-center spacing of 0.925 xcexcm.
Although it is less dependent on precise air void spacing, thereby resolving a problem presented by PGB fibers, the optical fiber 100 of the ""236 patent itself has shortcomings. First, because the core region 102 is solid glass material, the propagating light waves still xe2x80x9cseexe2x80x9d a substantial amount of glass as they travel down the fiber, and thereby still experience the loss, nonlinearities, etc., associated with travelling through that much glass material. Second, the first and second cladding features 104 and 106 have dimensions that are substantial fractions of the 1.55 xcexcm wavelength of light to be carried by the fiber 100 (54% and 44%, respectively, of the 1.55 xcexcm wavelength in vacuum, or 81% and 67%, respectively, of the 1.033 xcexcm wavelength in silica glass having a refractive index of 1.5.). In such case, the effective index of refraction may not be sufficiently uniform around any given circle at a given radius from the center, causing unwanted polarization, losses, or other undesired effects. Furthermore, because the air voids are relatively large and leave a substantial amount of solid glass in the cross section, the optical fiber 100 is still subject to microcrack propagation and unwanted polarizations that may be induced by external bending, pinching, or squeezing of the optical fiber. Finally, the capillaries formed by the air voids run continuously through the fiber from end-to-end. In practical installations in which water or other fluid might accidentally enter the optical fiber at a single point, there is nothing to prevent catastrophic failure of the optical fiber due to flooding of entire capillaries.
Accordingly, it would be desirable to provide an improved optical fiber having reduced signal attenuation for increased efficiency in long-distance fiber optic communications.
It would be further desirable to provide an optical fiber having reduced dispersion effects and nonlinear effects for increased efficiency in long-distance fiber optic communications.
It would be still further desirable to provide an improved fiber optic structure that is more robust than prior art fibers to external bending, pinching, or squeezing of the fiber optic cable.
It would be even further desirable to provide, in a microstructured optical fiber having capillaries formed by air voids, a structural improvement that prevents catastrophic flooding of the optical fiber upon accidental introduction of fluid into the fiber at a given point.
Other desirable features, effects and results will become apparent from the disclosure below.
In accordance with a preferred embodiment, a microstructured optical fiber is provided comprising a core region and a cladding region, the core region being made of a core material such as fused silica into which is formed a plurality of void regions that are elongated and parallel to a center longitudinal axis. The void regions are preferably filled with air, vacuum, or an inert gas, but may be filled a variety of liquids or solids. When the core region is viewed in cross-section, the void regions occupy a substantial first percentage of the area of the core region. The cladding region is likewise made of a cladding material, which is usually the same as the core material, into which is formed a plurality of void regions that are elongated and parallel to the center longitudinal axis. When the cladding region is viewed in cross-section, the void regions occupy a substantial second percentage of the area of the cladding region. In accordance with a preferred embodiment, the first percentage of void area in the core region is less than the second percentage of void area in the cladding region, whereby the cladding region has a lower effective index of refraction than the core region. This effective refractive index difference allows for propagation of light waves through the optical fiber by an index-guiding effect.
In a preferred embodiment, there is an approximately greater than 1:1 ratio of void area to material area in both the core region and cladding region of the microstructured optical fiber. Because the light waves xe2x80x9cseexe2x80x9d (or interact with) less glass material as they propagate down the longitudinal axis of the fiber, there is increased transmission efficiency as compared to conventional index-guiding optical fibers in the form of reduced signal attenuation, reduced dispersion effects, and reduced nonlinear effects.
According to a preferred embodiment, microstructural dimensions in the core and cladding are kept very small as compared to the wavelength of light. For propagation of 1.55 xcexcm (micrometer) light, for a substantial length (over 99%) of the optical fiber, the average cross-sectional dimension of the void regions preferably should be no greater than 0.1 xcexcm, which is less than one-tenth of the wavelength, and no substantial number of void regions should have a cross-sectional dimension greater than 0.3 xcexcm. These dimensions are preferred due to the detrimental effects of large void sizes, such as non-uniform distribution of the effective refractive index and greater index variations due to the large void sizes, which may result in optical modal variations. However, for certain portions of the fiber, at points of slicing or the beginning or the end of the fiber, large voids may be introduced to change the modal pattern of the light in the fiber for mode matching to other optical elements. Advantageously, due to the small microstructural dimensions of the preferred embodiments, the incident light waves xe2x80x9cseexe2x80x9d a uniformly distributed effective index of refraction at any particular location of the fiber, thereby avoiding adverse effects due to localized variations that can occur with the larger microstructural dimensions of the prior art. The cross-sectional positioning of the void regions may be amorphous or periodic, provided that the effective index of refraction is uniformly distributed at any particular location. The microstructured optical fiber may comprise a sets of capillary-like tubes or other structures that are bundled together, or may alternatively comprise a unitary piece of fused silica or other fiber optic material into which the void regions are etched out.
In addition to having increased transmission efficiency, a microstructured optical fiber in accordance with the preferred embodiments exhibits improved physical durability and robustness. Because of the very small size of the void regions, and the accordingly higher spatial density of voids, if the optical fiber is pinched or squeezed at a given point, the outer void regions preferably will deform to absorb the pinching pressure and leave the inner voids substantially unchanged. This preferably will avoid unwanted polarizations or other adverse effects exhibited by solid fused silica when subjected to such stresses in conventional optical fibers. Moreover, the small void regions serve to mechanically inhibit the propagation of microcracks into the cladding or core from the outer perimeter of the fiber, providing increased robustness to external bending of the optical fiber.
Further physical durability is achieved where, in accordance with a preferred embodiment, the void regions are intermittently pinched off in their longitudinal directions as part of their manufacturing process. This prevents capillary-effect flooding of the optical fiber if there is an unintended introduction of water, water vapor, or other fluid or fluid vapor into the fiber. In accordance with a preferred embodiment, the longitudinal positionings of the intermittent pinches are randomly distributed among the void regions, such that light waves propagating down the fiber are not substantially affected by the presence of the intermittent pinches in the air voids. In accordance with an alternative embodiment, the longitudinal positionings of the intermittent pinches may be distributed in a periodic fashion such that reflections would be reduced by interference effects.